Interferometers have historically enjoyed a wide variety of applications for analyzing material properties. For example, as incorporated in a Fourier transform spectrometer, an interferometer may be used in the medical field to detect and measure various constituents of body tissues and fluids. Interferometer spectrometers are particularly useful in the medical field because they allow for relatively non-invasive measurement techniques, as compared to prior art techniques which require tissue and/or fluid sampling by physically removing the sample from the patient.
The ability to perform relatively non-invasive procedures in the measurement of body tissue and/or fluid characteristics provides a tremendous advantage over the relatively invasive procedures of the prior art. For example, U.S. Pat. No. 5,830,132 to Robinson describes a robust and accurate non-invasive analyte monitor utilizing a light dispersion device such as an interferometer spectrometer for the measurement of blood constituents including glucose, alcohol, BUN (blood urea nitrogen), bilirubin, hemoglobin, creatin, cholesterol, and electrolytes. Another example of a non-invasive analyte monitor is disclosed in U.S. Pat. No. 5,655,530 to Messerschmidt. The system and method of Messerschmidt '530 utilizes spectrographic techniques in conjunction with an improved optical interface. As applied to the measurement of blood glucose levels, the analyte monitors disclosed in Messerschmidt '530 and Robinson '132 provide a diabetic patient with the opportunity to greatly improve control of the disease by more frequent or even continuous glucose monitoring, which translates into a reduction in diabetic related complications, an increase in patient comfort, an increase in life expectancy, and an overall improvement in daily life coping with the disease.
Continuous or at least more frequent glucose monitoring is achieved by eliminating the necessity to obtain a blood or other fluid sample. Practically speaking, a blood sample may not be obtained on a continuous basis nor at a sufficient frequency due to obvious reasons associated with risk of infection, patient discomfort, and patient lifestyle. The analyte monitors disclosed in Messerschmidt '530 and Robinson '132 overcome these obstacles by providing a non-invasive and painless means to measure blood glucose levels, thereby eliminating risk of infection and patient discomfort.
From the foregoing, it is apparent that interferometer spectrometers may have a significant impact on continuing efforts to improve the health of chronically ill patients, such as diabetics, by providing a significant improvement over prior art systems and methods of analyzing bodily tissues and/or fluids. However, this and many other applications require, for practical purposes, a relatively compact and robust interferometer. Specifically, a practical application of an interferometer spectrometer requires that the interferometer be compact such that it is portable and robust such that it is able to withstand the abuse of everyday use. Unfortunately, the prior art interferometers are, relatively speaking, neither sufficiently compact nor sufficiently robust to provide a practical interferometer spectrometer for portable use.
Probably the most famous interferometer design is the Michelson interferometer, which is commonly used for Fourier transform spectroscopy. A form of Michelson interferometer commonly used for Fourier transform spectroscopy includes six (6) basic components, namely, a collimated light source, a beam splitter, a compensator, a fixed flat end mirror, a movable flat end mirror, and a light detector. The movable end mirror may be translated along an axis perpendicular to its surface to generate a series of optical path length differences (OPD) used to measure the spectral properties of the light.
In use, light emitted from the light source strikes the beam splitter, which partially reflects and partially transmits the light therethrough. The reflected beam travels to the movable mirror and is reflected back through the beam splitter toward the detector. The transmitted beam travels through the compensator plate (same thickness and material as the beam splitter plate) to the fixed end mirror and is reflected back through the compensator plate, reflected off of the beam splitter and toward the light detector.
As mentioned previously, the movable mirror may be translated back and forth with a finely calibrated screw adjustment, or the like, to generate an optical path length difference (OPD) or cause retardation such that the recombined beam forms an interference pattern, commonly referred to as an interferogram. Retardation is the OPD between a pair of output rays originating from a single input ray. By observing the interference pattern, and measuring the distance the movable mirror is translated, the wavelength of the light provided by the light source may be determined. Further, changes in wavelength may be measured to determine the index of refraction of test samples which may then be used to identify the material and characteristics of the test sample. Further yet, by observing the interference pattern at various wavelengths, the amount of light absorbed by test sample may be measured, which is indicative of the material and properties of the test sample.
Although the Michelson interferometer is extremely useful, it tends to be relatively sensitive to alignment of its various components. In particular, a tilt error is created by a change in the angle of the beam splitter, the fixed-end mirror, or the movable-end mirror relative to the other components. Tilt error may be defined as a deviation from strict parallelism of a pair of output rays originating from a single input ray. The effect of a tilt error is to reduce the modulation efficiency of the interferometer, in a wavelength dependent manner, causing a spectral calibration error. For example, a change in angle of an end mirror, corresponding to an edge displacement (relative movement of opposite edges of the end mirror) by less than five percent (5%) of the wavelength of the light, causes an unacceptable change in calibration of the interferometer. This type of alignment sensitivity is particularly difficult to eliminate with regard to the movable end mirror.
Attempts have been made, with limited success, to eliminate the tilt error of the Michelson interferometer by replacing the flat end mirror with retroreflectors as described by W. H. Steel, “Interferometers for Fourier Spectroscopy,” Aspen International Conference on Fourier Spectroscopy, pp. 43-53 (1970). Although replacing the flat end mirrors with retroreflectors, such as cube-corner type or “cat's-eye” type retroreflectors, eliminate tilt error, a shear error may be caused by the lateral displacement of either retroreflector or a tilt of the beam splitter. Shear error is the lateral displacement of one light path relative to the other light path which causes a wavelength dependent reduction in the modulation efficiency of the interferometer. Shear error may be defined as a lateral separation of a pair of parallel output rays originating from a single input ray when the optical path difference (OPD) between the two rays is zero. Even a relatively small shear error on the order of a few wavelengths of light may be detrimental to the calibration of the interferometer.
Other attempts have been made to improve on the Michelson interferometer design in an effort to reduce alignment sensitivity of the components. For example, the Folded Jamin design provides a relatively stable design utilizing a relatively thick beam splitter plate and a rocking mirror as described by L. Mertz, “Transformations in Optics,” page 50 (1965). Although the Folded Jamin design reduces component alignment sensitivity, an exact ray trace analysis of the design demonstrates that the allowable field of view (FOV) is relatively small, particularly as compared to the FOV of the Michelson interferometer. A relatively small FOV renders the Jamin interferometer unsuitable for Fourier transform spectroscopy, particularly when the signal-to-noise ratio must be optimized through the use of a light source of a large angular subtense.
Further attempts have been made to reduce the alignment sensitivity of the Michelson interferometer by rotating the interferometer components as a group to generate the OPD. For example, U.S. Pat. No. 4,684,255 to Ford and the article by R.S. Sternberg and J.F. James “A New Type Of Michelson Interference Spectrometer,” J. Sci. Instru., Vol. 41 (1964) pp. 225-226, describe interferometers wherein the OPD is generated by rotating four components as a group. Another example is disclosed in U.S. Pat. No. 5,537,208 to Bertram et al. which describes an interferometer wherein the OPD is generated by rotating two mirrors in parallel. Although tilt error and shear error are eliminated by these designs to the extent that the components are rotated as a group with no relative movement therebetween, tilt and shear error may be caused by an incorrectly positioned component as constructed. As such, these designs inherently rely on the precise positioning and mounting of the components, as constructed and maintained thereafter, to eliminate tilt and shear error. For example, European Patent Application 0681166 A1 proposes the use of optically flat and parallel spacers to establish optical contact between the critical components and thereby maintain the precise position of the components. However, such component mounting techniques are relatively costly to implement.
In sum, many of the interferometer spectrometers proposed in the prior art are sensitive to relative alignment between the critical components, and thus are susceptible to tilt error and/or shear error. Attempts to reduce the alignment sensitivity of the various components have been met with limited success. Specifically, interferometer spectrometers of the prior art that reduce tilt and/or shear error have done so by compromising other performance aspects of the design and by increasing manufacturing costs.